This was driving me crazy, so I made a few amendments as part of an exercise (I’m learning web development), but also so I could stop having random things blind me or be unreadable. It’s not perfect by any means (code snippits are a especially a little wonky if they’re given a language, so syntax highlighting will be mostly overwritten to ensure readability), but it works. Sharing the amendment in case someone finds it useful. Sorry I’m not more of an artist.

Works great for WaniKani Forums, though reviews and lessons look a bit odd. But you can choose on a site by site basis whether to turn on dark mode or not. You can have it on for all sites and white-list the ones it doesn’t work well with, or off for all sites and activate it for just the sites you want.